In the context of an installation module consisting of a metallic support structure, German utility model 88 00 349, for example, discloses that the assembly space, rather than being filled with masonry, be covered by an apron in the form an apertured steel plate positioned substantially flush with the front wall of the module. This apron is formed with a recess for passage of a drain pipe of the installation module and is adapted to be fixed, on the one hand, to the bottom side of the installation module and, on the other hand, to the bare floor by way of supports which are adjustable in height.
With foamed installation modules it was attempted to obtain a uniform surface for plastering or tiling also by using a foamed apron to cover the assembly space, conveniently made of the same foam compound as the installation module. Yet that caused problems regarding strength and dimensional stability. In a water closet installation module the apron must withstand the standard load (approximately 400 kg) of the water closet suspended from the wall. Sufficient strength has hitherto been obtainable only by use of an apron that is correspondingly thick. However, the space available between the front face of the module and the waste water draining pipe elbow is limited. In spite of greater thickness of the apron, the risk remains that parts will break out, especially so in the area of the fastening connections and at the edges. What has remained unsolved so far is that foamed aprons tend to distort easily.